1. Field of Invention
The present invention relates to a field of optical fiber sensing technology, and more particularly to a long distance polarization and phase-sensitive optical time-domain reflectometry having optical fiber random lasers as a distributed pump source.
2. Description of Related Arts
In the distributed optical fiber transmission technology, the optical fibers are both the transmission medium and the sensing elements; meanwhile, the optical fibers obtain the spatial distribution states of the measured parameters at a whole length of the optical fibers and the information changes of the measured parameters over time, so it is very suitable for the optical fibers to be applied in the safety and protection of long-distance optical cables and the peripheral security based on the optical cables. Because the phase and the polarization state of the light pulses transmitted in the optical fibers are very sensitive to the perturbation of the optical fiber lines, the phase-sensitive optical time-domain reflectometry (-OTDR) and the polarization optical time domain reflectometry (POTDR) which are made according to the feature that the phase and the polarization state of the light pulse transmitted in the optical fibers changes with the line state are able to detect whether the optical fiber lines are perturbed or not, which facilitates the judgment of whether there is anybody approaching the communications cables, so as to accomplish early warning and avoid the destruction of the optical cables to a greatest extent.
The conventional phase-sensitive and polarization optical time-domain reflectometries in a front-end concentrated amplification manner are unable to ensure the measurement accuracy of the optical fiber back-end, which is explained as follows. Firstly, the light power peak of the signal light is prevented to be too high, otherwise unstable modulation, decrease of frequency spectrum broadening caused by the self-phase modulation or decrease of measurement accuracy may be caused. Secondly, the optical fiber loss and the pump consumption effect can affect the measurement resolution of the optical fiber back-end. Thirdly, along with the increasing requirement of spatial resolution, the used pulse width is increasingly narrow, which results in the decrease of the energy carried by the signal pulse to further result in the decrease of the measurement accuracy.
Although the conventional first-order distributed Raman amplification technology is able to improve the spatial distribution uniformity of the optical signals to some extent and ensure the entire consistency of the measurement accuracy, the relative intensity noise RIN of the Raman pump source is usually larger than −100 dBc/Hz; the RIN transfer from the pump to the scattered light becomes an important factor restricting the extension of the sensing distance and the accuracy improvement.